The present invention relates to a method and apparatus for optically recording and reproducing digital information.
A digital information recording medium (or optical recording medium) is known in the art which can record, reproduce or erase digital information signals by applying a laser light on a recording film formed on a substrate to heat the film locally and changing optical or magnetic properties of the recording film. With such an information recording medium, a recording density more than ten times as high as that of a magnetic recording medium can be attained.
The recording density of such an optical recording medium (the density is the number of bits per unit area to be recorded on the medium) is determined by the focussed spot diameter. The smaller the diameter is, the higher the recording density becomes. The laser beam spot diameter of an optical recording medium currently used for information recording and reproducing is about 1 micron which is almost its limit value considering short wavelengths of a laser beam and numerical apertures of an objective lens. Therefore, it cannot be expected for higher recording density.
To solve this problem, a multiple light-wavelength memory has been proposed using PHB (photochemical hole burning) phenomenon, as disclosed in the specification of U.S. Pat. No. 4,101,976. The PHB phenomenon is a kind of bleaching caused by wavelength-selective photochemical reactions of a material. Known such materials include phtalocyanine (H.sub.2 PC) dispersed polymethyl methacrylate (PMMA) and the like.
The PHB phenomenon will be described briefly with reference to FIG. 1.
The spectral characteristic of light absorption and wavelength of such materials can be represented by an absorption band with a not-uniform width .DELTA..omega..sub.i comprising a plurality of absorption bands WT with an uniform width .DELTA..omega..sub.h, as shown in FIG. 1(A). If such a material is exposed to laser lights having wavelengths .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3 each suitable for a certain absorption band WT having a wave-length .lambda..sub.l, only those molecules having energy whose quantity is in the range of photon-energies at opposite side absorption bands WT of the laser beams each having a photon-energy of h.nu..sub.l =h.sub.c /.lambda..sub.l, resonantly absorb light and become excited. These excited molecules are transformed into a light-induced substance having a different energy without affecting other molecules. As a result, as shown in FIG. 1(C) of the spectral characteristic, narrow optical holes appear respectively at wavelengths .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3, having a width equal to the width .DELTA..omega..sub.h of the absorption band WT and a lifetime determined by the stability of the light-induced substance. The absorption power at the hole is smaller than at other positions and varies with the intensity of a laser beam. If a white light is applied to a thin film made of such a material, signals are detected as shown in FIG. 1(D) at the wave-lengths .lambda..sub.1, .lambda..sub.2 and .lambda..sub.3 corresponding to a large transmissivity. Therefore, a monochrome light of wavelength .lambda..sub.1 for example is scanned for reproduction, only the information having been recorded with a laser beam of wavelength .lambda..sub.1 can be obtained.
The prior art disclosed in the above-mentioned U.S. Patent specification utilizes such phenomenon. According to this prior art, a laser beam of wavelength .lambda..sub.1 is first used for recording information on the entire surface of an information recording medium. Next, a laser beam of wavelength .lambda..sub.1 is used for recording. Similarly, recording information on the entire surface of the medium is performed switching the wavelength of a laser beam. Switching the wavelength of a laser beam is also performed in case of reproducing the information. Switching the wavelength during reproduction is accompanied with some problems in practical use. Since the transmissivity of a laser beam varies depending only on the absorption power of the absorption band WT concerned, if information on the thin film is to be reproduced using a laser beam of wavelength .lambda..sub.1, only the information having been recorded with a laser beam of wavelength .lambda..sub.1 is reproduced.
Therefore, recording areas for laser beams each having a wavelength of .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and so on may be superposed so that information bits by laser beams may be superposed thereby enabling multiple recording.
According to the conventional multiple light-wavelength recording which records superposing information bits by varying the wavelength of a laser beam, the recording density of an information recording medium becomes a multiple of the recording density without superposing bits, by the number of wavelengths of a laser beam to be varied, thus enabling a high density recording. The quantity of the recording density multiplied by the area usable for recording on an information recording medium, i.e., the maximum information amount which can be recorded, is called a recording capacity.
Apart from the above, use of an information recording medium as an image file memory has recently been adopted in common and it has been desired to improve the quality of reproduced signals such as image signals. To this end, a broad bandwidth for reproduced signals is now under consideration. To realize a broad handwidth, it is necessary to speed up a transfer rate, which is defined as the information amount which can be recorded or reproduced per unit time, or in other words the number of information signal samplings which can be conducted for recording or reproducing per unit time. The higher the transfer rate is, the broader the recording bandwidth of information signals can be made. This transfer rate is related to the spot diameter of a laser beam. That is, the smaller the laser beam spot diameter is, the smaller the area occupied by an information bit becomes. If information bits are sequentially placed close, the number of information bits to be recorded to reproduced by unit time increases. Thus, the number of information signal samplings for recording and reproducing increases to thereby make the transfer rate higher. In view of this, it can be said that the higher the recording density is, the higher the transfer rate becomes.
The prior art disclosed in the above-noted U.S. Patent specification realizes a high density recording using the PHB phenomenon. According to the prior art, the wavelength of a laser beam is changed for each recording on the entire surface of an information recording medium. Thus, it is intended that the time while information signals can be recorded is made longer by multi light-wavelength, or in other words it is intended that the apparent recording capacity of an information recording medium is made large.
The transfer rate according to the prior art is determined by the dimension of an information bit because the information bits, which are sequentially generated with a same wavelength laser beam, are disposed side by side without interposing upon another similarly to the conventional art. However, as previously described, the laser beam spot diameter is now at its limit value of about 1 micron so that the dimension of an information bit is also neat its limit value. Thus, improvement on the transfer rate cannot be expected.